Skin cancers, including squamous cell (SCC) and basal cell carcinoma (BCC), are the most common of all human cancers. Although seldom lethal, skin cancers are a huge economic burden to the healthcare system (fifth most expensive, behind breast, colon, lung, and prostate cancer). In addition, surgical treatment of skin cancer often leads to unsightly, hypertrophic scars. Photodynamic Therapy (PDT) is a non-scarring treatment that targets cancer cells in a two step process: (1) the tumor-selective accumulation of a photosensitizing drug, and (2) illumination with strong visible light. The PDT regimen most frequently employed uses aminolevulinic acid (ALA) as a prodrug, which is selectively taken up by tumor cells and converted into protoporphyrin IX (PpIX); the latter is the actual photosensitizer that absorbs light. Despite widespread acceptance of PDT in European countries for treating SCC and BCC, due to its good cosmetic results and lower costs, the only FDA- approved use of PDT in the U.S.A. is for the treatment of pre-cancers (actinic keratoses). One reason for this, aside from poor economic incentives for U.S. physicians to use a lower-reimbursing modality, is the fact that PDT is not quite as effective as surgical excision for deep nodular BCC or invasive SCC. As a way to improve ALA-PDT efficacy, we recently showed in SCC tumors in mice that pretreatment with calcitriol, the active form of Vitamin D3 (VD3), improves PpIX accumulation and enhances tumor-specific killing by ALA-PDT. The approach with calcitriol has a potential problem, however, because calcitriol, if overdosed, can cause toxicity (hypercalcemia). To circumvent this, we have preliminary data to show that cholecalciferol (D3) as a short-term dietary supplement can in fact significantly boost production of PpIX in skin tumors, to nearly the same extent as systemic pretreatment with calcitriol, but without the hypercalcemic risk. The overall goal of the current proposal is to test whether oral Vitamin D3 (cholecalciferol), used in a combination approach with PDT (cPDT), is a safe and viable way to enhance the efficacy of treatment for skin cancer. Two experimental aims will be pursued in mouse models of cutaneous SCC (human xenograft) and basal cell carcinoma (murine BCC). Aim 1 will determine the mechanisms by which oral D3 causes specific PpIX accumulation in the tumor tissue. Experiments will test whether conversion of serum 25-OH D3 to calcitriol occurs locally in skin tumors, and whether the Vitamin D receptor (VDR) is required for PpIX induction. Aim 2 will establish the proper dose range and time course for oral D3 to achieve safe and effective PpIX photosensitization. In addition, since the majority of people in the U.S.A. are known to be Vitamin D3 deficient, Aim 2 will ask whether a state of baseline VD3 deficiency affects the ability of administered cholecalciferol to enhance PpIX levels and PDT efficacy. Successful completion of this work should provide sufficient information to guide the design of future clinical trials designed to treat patients with SCC, BCC, and metastatic epithelia carcinomas. The potential benefit to public health would be a safe, effective, non-scarring alternative to surgery for treatment of cancer.